linux-kernel - Re: [BUG 2.6.31-rc1] HIGHMEM64G causes hang in PCI init on 32-bit x86

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Message-ID: <alpine.LFD.2.01.0906291820060.3605@localhost.localdomain>
Date:	Mon, 29 Jun 2009 18:26:29 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Yinghai Lu <yinghai@...nel.org>
cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Mikael Pettersson <mikpe@...uu.se>,
	Matthew Wilcox <matthew@....cx>,
	Grant Grundler <grundler@...isc-linux.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-pci@...r.kernel.org
Subject: Re: [BUG 2.6.31-rc1] HIGHMEM64G causes hang in PCI init on 32-bit
 x86

On Mon, 29 Jun 2009, Yinghai Lu wrote:
> +		end = round_up(start, ram_alignment(start)) - 1;
> +		if (start > (resource_size_t)end)
>  			continue;
> -		reserve_region_with_split(&iomem_resource, start,
> -						  end - 1, "RAM buffer");
> +		reserve_region_with_split(&iomem_resource, (resource_size_t)start,
> +					  (resource_size_t)end, "RAM buffer");

Hmm. You shouldn't need the casts with reserve_region_with_split(), and 
they just make things uglier.

Also, I wonder if we should do something like this instead

	#define MAX_RESOURCE_SIZE ((resource_size_t)-1)

	...
	end = round_up(start, ram_alignment(start)) - 1;
	if (end > MAX_RESOURCE_SIZE)
		end = MAX_RESOURCE_SIZE;
	if (start > end)
		continue;

Because otherwise we'll just be ignoring resources that cross the resource 
size boundary, which sounds wrong.

We _could_ have a RAM resource that crosses the 4GB boundary, after all.

Yeah, it doesn't happen much in practice, because usually the 3G-4G range 
is left for PCI mappings etc, so we might never hit this in practice, but 
still, this sounds like a more correct thing to do.

It also avoids the cast. We simply cap the end to the max that 
'resource_size_t' can hold.

That said, I have to admit that I'm getting tired of these bugs that only 
happen when we have a 32-bit resource_size_t. So I can understand the 
attraction to just forcing it to 64-bit and forgetting about these 
irritating issues.

		Linus
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